Currently, certain RF legs, meaning: “constant radius between two fixes” and certain AF legs (Arc to a fix) are present in the navigation database of certain aircraft, notably in the majority of Airbus aircraft.
The AF and RF legs correspond to substantially circular portions of trajectory which form circular arcs. They are generally included in a flight plan of an aircraft by knowing the entry and exit points of the trajectory portions to be flown. According to the types of legs, they can be defined for example by a point corresponding to the centre of a circle, the point being charted in terms of latitude and longitude with respect to a known aerial beacon, and by two points situated at the ends of the arc, these are an entry point and an exit point of the trajectory portion corresponding to the circular arc.
The AF and RF legs can be included in a flight plan of the FMS when inserting a procedure corresponding to a departure or arrival of the aircraft for example. According to the beacons, the databases and the air routes, the RF and AF legs are generally predefined in the FMS. In such a case, it is simple with the aid of the FMS to fly certain portions corresponding to these legs in an automatic manner.
On the other hand, in a manner not planned in the flight plan of an aircraft's FMS, it may happen that in the terminal phase of a mission, air traffic control gives the aircraft presets to fly arcs at a constant distance with respect to a given point. In this case, this preset is often de-correlated from a terminal procedure existing in the navigation database and the arc does not exist in this database.
This may for example involve a request from air traffic control to an aircraft, outside of a defined procedure, to fly an arc around an aerial beacon, also called a DME arc in aeronautical terminology. This procedure may be necessary to allow an aircraft to align itself on the appropriate approach axis so as to start a final approach.
Moreover, this type of procedure can be undertaken on the initiative of the pilot who wishes to embark on an approach procedure. The latter case usually corresponds to military or general aviation flights operating on small aerodromes.
Currently, when the crew wishes to fly an arc corresponding to an AF or RF leg that is not planned by the FMS, two solutions are possible.
In a first case, the crew must create a series of waypoints of the flight plan that one wishes to fly. Waypoints, in aeronautical terminology, are points defined in the navigation database of the aircraft. These waypoints are created one by one up to the construction of a circular arc. This solution remains arduous since it is performed manually, moreover it is approximate.
In a second case the crew can use functions of the FMS making it possible to plot circular arcs but these functions are not intended for planning portions of a flight plan. The latter solution presents the disadvantage of having to fly the trajectory manually, trying to follow as closely as possible the circular arc forming the trajectory.